Method for the production of a structure element

ABSTRACT

The invention relates to a process for producing a structural element, having at least one cavity-forming half-shell, comprising at least two lips which together form at least one flange, and is distinguished by the fact that at least the lips are provided, at their contact surfaces, with a foam semi-finished product, that the lips are brought together with the aid of a clamping force, that the foam semi-finished product is activated, and the lips are welded together by the activated foam semi-finished product.

This application claims the priority of German application no. 101 43 557.6, filed Sep. 6, 2001, and PCT International Patent Application No. PCT/EP02/08976, filed Aug. 10, 2002, the disclosure of which is expressly incorporated by reference herein.

The invention relates to a process for producing a structural element having at least one cavity-forming half-shell with two lips which together form at least one flange.

In view of the increased safety and strength demands imposed on motor vehicles, hollow structural elements comprising foam elements are increasingly being used.

One problem with reinforcing structural elements by means of foam elements is the accurate introduction of the foam elements into the cavity in the structural elements. U.S. Pat. No. 5,806,919 describes a structural element reinforced by a foam element. The foam element includes two types of foam, a reinforcing foam with a high density, which bears directly against the surface of the structural element, and a core foam with a low density, which is completely surrounded by the reinforcing foam.

The combined foam element, in ready-structured form, is placed into a half-shell of the structural element. The half-shell is closed up by a further half-shell and the two half-shells are welded together at flanges. Suitable toothed engagement anchors the foam element in the cavity in the structural element.

A structural element of the same general type is described in EP 1 052 162 A2. Components of this type are afflicted with the fundamental drawback that the foam elements have to be shaped and then placed into the structural element. In practice, this is only possible with structural parts of multipart configuration which are permanently closed up, for example by welding, after the foam element has been put in place. The positioning accuracy of the foam element is in this case often insufficient, which is to the detriment of the strength of the element. Moreover, the joining of the half-shells, which is generally carried out by welding, means a further, technically complex working step.

An alternative is for the foam element to be foamed directly inside the structural element. A process of this type is described in DE 199 12 618 A1, according to which a component is partially reinforced. The components described in DE 199 12 618 A1 are of single-part structure, but their geometry is relatively simple, so that the foam semi-finished product can be introduced into the structural element after shaping. After the foaming process, the foam fills the corresponding cavity and therefore forms the reinforcement and strengthening.

The structural element described in DE 199 12 618 A1 is shaped by hydroforming or similar techniques, which lead to closed structural elements. However, the process described in that document cannot be used to form structural elements from sheet-metal semi-finished products (or cast semi-finished products), since these products have to be joined so as to form a cavity.

The invention is based on the object of providing a process for joining hollow structural elements which are produced from one or more open semi-finished products and can be reinforced by a solid foam, which is more favorable than the processes which have been described in the prior art.

According to the process of the invention, at least one half-shell is shaped so as to form a cavity and is closed up by means of at least two lips which together form a flange. The process is distinguished by the fact that a foam semi-finished product, which is liquefied by activation, preferably thermal activation, is located at least between the lips at the locations where the latter are in contact. During this activation, the lips are pressed together and are thereby welded together. Therefore, it is possible to dispense with the welding or adhesive bonding step which is otherwise required during the production of structural elements.

In this case, the metallic foam material has the function of a solder in the region of the lips. On account of the high pressure on the lips, the foam semi-finished product is liquefied, since there is no space available for it to expand. The join which is formed can be considered as a type of soldered joint.

To improve the formation of the soldered joints, the lips which are to be joined are coated with the same material as the foam semi-finished product, or a similar material.

In accordance with another embodiment, the activation of the foam semi-finished product is preferably carried out by thermal means. Further forms of activation, such as chemical activation, electrical activation or activation by radiation, are also advantageous if corresponding inactivated foams are used.

The process can particularly advantageously be employed if the structural element is simultaneously reinforced with a foam at the same time as being welded together. In this context, in turn, two variants of the process according to the invention are particularly expedient.

According to one embodiment, the foam semi-finished product is applied areally both between the lips of the flange and to the remaining surface of the half-shell on the cavity side. It is preferable for the areal foam semi-finished product to be applied to the surface of the half-shell (for example by being clamped or adhesively bonded to it) before the half-shell is shaped.

This is preferably effected by roll cladding. The foam semi-finished product is rolled together with a metal sheet from which the half-shell is formed. In this way, the foam semi-finished product is fixedly joined to the metal sheet, and the join is substantially retained even during the deformation. The thickness of the foam semi-finished product is such that after foaming the cavity in the structural element is at least filled with foam such that a desired reinforcing effect occurs (claim 6).

In a further advantageous embodiment of the process according to the invention, a sheet-like foam semi-finished product is placed into the cavity and clamped in place by the lips of the flange, preferably of two opposite flanges. The activation causes the foam semi-finished product in the cavity to be foamed so that it fills the cavity. Foaming between the lips is prevented, and the liquid foam material, as has already been described, welds the flange together.

The process according to the invention can be carried out in a forming die. The die may surround the structural element either completely or only in the region of the flanges. This ensures that the structural element is uniformly clamped shut and is therefore welded together uniformly. Moreover, the activation heat for the foaming step can be supplied via the die.

The foam semi-finished produced is preferably based on metals, in particular zinc or aluminum. It is usually composed of a powder of these materials which contains a foaming agent, e.g. titanium hydride. This mixture is pressed to form plates which ultimately produce the foam semi-finished product.

Foams based on zinc, in particular when filling steel with foam, offer the advantage of providing additional protection against corrosion, and moreover they can be processed at relatively low temperatures. Zinc foams are therefore suitable for reinforcing aluminum or magnesium structural elements. Foams based on aluminum have a particularly low density, which is advantageous in particular for lightweight structural applications. Foams based on plastic, e.g. epoxy resins or polyurethanes, are likewise expedient in the context of the invention, but the joins which are formed cannot be subjected to such high loads as if metal foams are used (claim 9).

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section through a structural element comprising two half-shells with roll-clad foam semi-finished product,

FIG. 2 shows the semi-finished product from FIG. 1 after foaming and welding,

FIG. 3 shows a cross section through a structural element comprising two half-shells and foam element,

FIG. 4 shows a cross section through a structural element comprising one half-shell with roll-clad foam semi-finished product.

DETAILED DESCRIPTION OF THE DRAWINGS

The structural element which is illustrated in cross section in FIG. 1 comprises two half-shells 2 and 4, which are connected to one another by lips 6, 8, 10 and 12 in the style of flanges. The lips are coated with an alloy based on zinc. The half-shells forms a cavity 13. Steel sheets from which the half-shells 2 and 4 are made have a roll-clad layer of a foam semi-finished product 14. The thickness of the foam semi-finished product 14 is approx. 3 mm, and the foam semi-finished product is based on zinc. The layer comprising the foam semi-finished product 14 on the half-shells 2 and 4 faces into the cavity 13.

For better alignment, the half-shells 2 and 4 are clamped into a die (not shown here), which completely surrounds the assembled half-shells 2 and 4. The action of force exerted by this die is indicated by the force arrows F. Heat is transferred to the half-shells 2 and 4 via the die (not shown), and these half-shells are heated up to a foaming temperature of the foam semi-finished product. The foaming temperature is in the region of the melting point of the foam semi-finished product 14, which in the case of a zinc foam as used here is approx. 420° C.

In the cavity 13, the foam semi-finished product 14 expands unimpeded and becomes the foam 18 (FIG. 2). In the regions between the lips 6, 8 and 10, 12, the expansion of the foam semi-finished product 14 is suppressed by the compressive force F. At these locations, the foam semi-finished product is locally melted and welds or solders the lips 6, 8 and 10, 12 together after cooling to form flange-like joins 16, 17. The cavity 13 has been completely filled by the foam 18 and reinforces the completed structural element 15.

The half-shells 22 and 24 illustrated in FIG. 3 are of similar structure to those shown in FIG. 1 and are clamped into the same die. The difference with respect to FIG. 1 is that the half-shells are not roll clad with the foam semi-finished product, but rather a plate-like foam semi-finished product 34 is clamped between the lips 26, 28 and 30, 32. This foam semi-finished product 34 passes through the center of the cavity 35. The foaming process is similar to that which has been illustrated in FIGS. 1 and 2. However, in accordance with the arrow directions indicated in FIG. 3, the expansion of the foam semi-finished product 34 takes place from the center of the cavity 35 outward, in the direction of the half-shells 22 and 24. The resulting structural element corresponds to that shown in FIG. 2.

FIG. 4 illustrates a further configuration of the invention similar to that shown in FIG. 1. Unlike in FIG. 1, there is just one half-shell 36, which is shaped in such a manner that it encloses a cavity 37. The lips 38, 40 are likewise pressed together by the abovementioned die (not shown here). Similarly to the half-shells 2 and 4 shown in FIG. 1, the half-shell 36 is also provided with a roll-clad foam semi-finished product 42. The foaming process takes place in the same way as in FIGS. 1 and 2.

As an alternative to the arrangements of the join which have been described, it is also possible for the lips to form a butt joint with one another or to run inward. It is essential to the invention for there to be a sufficient contact surface for the foam material to be able to weld or solder them together.

The structural element can in this case be of virtually any desired geometry. The structural element may be composed of more than two half-shells, and moreover it may have various chambers which subdivide the cavity.

Advantageous alternatives to the materials include a combination of steel sheets and aluminum foam; this pairing allows the weight of the structural element to be reduced compared to the zinc foam. Furthermore, a pairing of aluminum sheets and zinc foam is expedient, since the melting point and therefore the foaming temperature of zinc is lower than the melting point of aluminum sheets. A pairing of magnesium half-shells with a zinc foam is also expedient. Although magnesium does not have advantageous properties for processing as a metal sheet, castings or extruded parts made from magnesium (and from aluminum) are also suitable for use as half-shells in the process according to the invention.

The use of plastic foams is expedient in the context of the invention if the demands imposed on the join between the half-shells is lower, for example if the structural element is used in an assembly in which no significant loading is exerted on the join.

The process according to the invention is employed in particular in the automotive industry, in particular in bodywork. Suitable components for reinforcing in accordance with the invention include, inter alia, the A, B and C pillars, sills, frame side rails, doors and intersections between the pillars or doors. Components which have been reinforced in this manner are eminently suitable for use as crash bars, since a considerable amount of energy can be absorbed by the foam reinforcement.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1. A process for producing a structural element, having at least one cavity-forming half-shell, comprising at least two lips which together form at least one flange, wherein at least the lips are provided, at their contact surfaces, with a metallic foam semi-finished product, the lips are brought together with the aid of a clamping force, the foam semi-finished product is activated, and the lips are welded together by the activated foam semi-finished product.
 2. The process as claimed in claim 1, wherein the half-shells, at least at the lips, are coated with the metal of the foam semi-finished product.
 3. The process as claimed in claim 1, wherein the foam semi-finished product is activated by thermal means.
 4. The process as claimed in claim 1, wherein the foam semi-finished product is arranged between the lips and in the cavity, and the cavity (13, 35, 37) is at least partially filled with foam as a result of the activation of the foam semi-finished product.
 5. The process as claimed in claim 1, wherein the foam semi-finished product is arranged areally over the surface of the half-shell on the cavity side.
 6. The process as claimed in claim 4, wherein the foam semi-finished product is applied to the surface of a metal sheet by roll cladding and the metal sheet is deformed to form the half-shell.
 7. The process as claimed in claim 1, wherein a sheet-like foam semi-finished product is clamped between the lips of the at least one flange, and at least subregions of the foam semi-finished product are located freely inside the cavity.
 8. The process as claimed in claim 1, wherein the at least one half-shell is clamped into a die.
 9. The process as claimed in claim 1, wherein to the foam semi-finished product is based on metal, in particular zinc, tin or aluminum.
 10. A process for producing a structural element, comprising the steps of: providing at least one cavity-forming half-shell including at least two lips which together form at least one flange, wherein at least the lips are provided, at their contact surfaces, with a metallic foam semi-finished product; bringing together the lips with the aid of a clamping force; activating the foam semi-finished product; and fixing the lips together with the activated foam semi-finished product.
 11. The process as claimed in claim 10, wherein the half-shells, at least at the lips, are coated with the metal of the foam semi-finished product.
 12. The process as claimed in claim 10, wherein in the step of activating the foam semi-finished product, the foam semi-finished product is activated by thermal means.
 13. The process as claimed in claim 10, wherein the foam semi-finished product is arranged between the lips and in the cavity, and the cavity is at least partially filled with foam as a result of the activation of the foam semi-finished product.
 14. The process as claimed in claim 10, wherein the foam semi-finished product is arranged areally over the surface of the half-shell on the cavity side.
 15. The process as claimed in claim 13, wherein in the step of providing at least one cavity-forming half-shell, the foam semi-finished product is applied to the surface of a metal sheet by roll cladding and the metal sheet is deformed to form the half-shell.
 16. The process as claimed in claim 10, wherein in the step of providing at least one cavity-forming half-shell, a sheet-like foam semi-finished product is clamped between the lips of the at least one flange, and at least subregions of the foam semi-finished product are located freely inside the cavity.
 17. The process as claimed in claim 10, further comprising the step of: clamping the at least one half-shell into a die.
 18. The process as claimed in claim 10, wherein the foam semi-finished product is based on a metal.
 19. The process as claimed in claim 18, wherein the metal is at least one of zinc, tin and aluminum. 